1. Field of the Invention
The present invention relates to synthetic quartz glass ingots and synthetic quartz glasses from which can be made optical blanks of high optical homogeneity and low variation in light transmittance for optical elements, such as lenses, prisms, mirrors and windows, used with excimer lasers, and particularly ArF excimer lasers. The invention also relates to methods for producing such synthetic quartz glass ingots and synthetic quartz glasses.
2. Prior Art
The drive toward higher levels of integration in VLSI circuits has led to a need for submicron-scale exposure technology in the photolithographic systems used to form integrated circuit patterns on wafers. Light sources of increasingly shorter wavelength are being employed in exposure systems to carry out patterning to smaller linewidths. The i-line (wavelength, 365 nm), once the light source of choice in lithography steppers, has been largely supplanted by the KrF excimer laser (248 nm), and today ArF excimer lasers (193 nm) are starting to see industrial use. Lenses used in such steppers are required to have an outstanding ability to transmit ultraviolet light, good resistance to damage from UV light irradiation, and high homogeneity.
To avoid the presence of metallic impurities, which cause UV light absorption, transparent synthetic quartz glass for making such lenses and other optical elements is generally produced by feeding a high-purity silicone compound such as silicon tetrachloride in vapor form directly to an oxyhydrogen flame, flame hydrolyzing the vapor to form fine particles of silica, and depositing, melting and vitrifying the particles directly on a rotating heat-resistant substrate made of a material such as quartz glass.
Transparent synthetic quartz glass produced in this way has a good transmittance to short-wavelength radiation down to a wavelength of about 190 nm, and is used as a light-transmitting material for UV laser light, specifically i-line radiation, excimer laser light such as that from KrF (248 nm), XeCl (308 nm), XeBr (282 nm), XeF (351 and 353 nm) and ArF (193 nm) lasers, and the fourth harmonic (250 nm) of YAG lasers.
The new light absorption in the UV region that emerges when synthetic quartz glass is irradiated with UV light of intense energy such as an excimer laser is believed to be due to paramagnetic defects which arise from photoreactions caused by inherent defects within the synthetic quartz glass. Numerous instances of light absorption from paramagnetic defects have been identified in ESR and other spectra. Examples of such defects include E′ centers (Si.) and non-bridging oxygen radicals (Si—O.).
Thus, paramagnetic defects generally have optical absorption bands. When quartz glass is irradiated with UV light, absorption bands of concern in the UV wavelength region due to paramagnetic defects in the quartz glass include bands at 215 nm and, though not yet precisely identified, 260 nm, both due to E′ centers (Si.). In some cases, these absorption bands are relatively broad and absorption is strong. For example, when quartz glass is used as an ArF excimer laser or KrF excimer laser light-transmitting material, such absorption can be a major problem.
The inherent defects in synthetic quartz glass that give rise to paramagnetic defects include non-SiO2 structures such as Si—OH and Si—Cl, oxygen deficient structures such as Si—Si, and oxygen surplus structures such as Si—O—O—Si.
One way to suppress paramagnetic defects is taught in JP-A 6-199532, which discloses a method wherein a chlorine-free alkoxysilane such as tetramethoxysilane is used as the silica-forming compound to prevent Si—Cl structures, a type of paramagnetic defect-inducing structural defect, from being included within the glass.
The presence of at least a given concentration of hydrogen molecules in quartz glass is known to discourage the formation of E′ centers (Si.), one type of oxygen defect, thus enhancing the laser durability of the glass.
ArF excimer laser light causes a level of damage in quartz glass which is several times greater than that by KrF excimer laser light. Hence, quartz glass for ArF excimer laser applications must have a hydrogen molecule concentration several times higher than that of quartz glass for KrF excimer laser applications.
A method of controlling the hydrogen molecule concentration in synthetic quartz glass is described in the prior art (JP-A 6-305736). The current practice is to adjust the concentration of hydrogen molecules in glass in accordance with the intended ArF laser energy use conditions.
Extensive research has thus been devoted to improving the laser durability of quartz glass as the light sources used in photolithography have become of increasingly shorter wavelength and more intense (as illustrated by the shift from i-line irradiation to excimer laser light).
Such shifts to shorter wavelength lithography are also changing expectations with regard to optical elements used in exposure tools (e.g., lenses, windows, prisms). One distinct trend recently has been toward a higher numerical aperture in the projection lens materials used in exposure tools. The steady rise in the aperture of the lens material has been accompanied by a need for lens materials of greater (more precise) homogeneity. In addition to the homogeneity of the index of refraction, a very important problem concerning ArF excimer lasers has been that of reducing birefringence. Quartz glass in particular, when exposed to light having a wavelength shorter than 200 nm, has been found to experience a loss in the constancy of its photoelastic coefficient, with sudden, large changes in value. At a wavelength of 193 nm, the photoelastic coefficient rises to a value about 1.5 times greater than that at a wavelength of 633 nm. The influence of birefringence on the resolution is thus even larger now than before. Accordingly, as with the need to achieve greater refractive index homogeneity, it has become essential also to reduce birefringence to the lowest possible level.
Hydroxyl group concentration, chlorine concentration and fictive temperature are well-known parameters in quartz glass that determine the homogeneity of the refractive index distribution. By suitably combining the distribution profiles of these parameters within the glass, it has been possible to reduce the refractive index distribution Δn to a value on the order of 1×10−6. However, in methods that increase the refractive index homogeneity by combining the distribution profiles of these parameters in such a way as to cancel out their respective influences on the refractive index, particularly with the use of chlorine-free synthetic quartz glass having a hydroxyl group concentration of more than 1,000 ppm to confer ArF excimer laser resistance, the following observations have been made:    (1) Synthetic quartz glass ingots which are chlorine-free and have a high hydroxyl group concentration tend to have a higher viscosity at elevated temperatures, making the glass ingot more difficult to homogenize.    (2) A high degree of homogeneity is required in the large-aperture products that have emerged from recent efforts to achieve higher levels of circuit integration by increasing numerical aperture.    (3) Because little has been done in the past to minimize birefringence, a certain degree of birefringence is unavoidable.Hence, although synthetic quartz glasses obtained in this way can be used to make optical components for KrF excimer lasers, they do not meet the much stiffer requirements for refractive index distribution Δn and birefringence associated with ArF excimer laser-related applications.